1. Field of the Invention
This invention relates to innerseals and capliners for containers.
2. Discussion of the Prior Art
Innerseals for containers are typically made of a disk-shaped sheet of a metallic foil/polymeric layer laminate which is sealed to the mouth of the container by means of induction heating.
Typical sheet stock for preparing such innerseals is made by bonding a sheet of the foil/polymeric layer laminate to a sheet of pulpboard by means of a layer of wax. The wax layer holds the laminate and pulpboard sheet together, so that the innerseal can be formed from the innerseal sheet stock in a single pass on conventional caplining equipment. After the sheet stock for preparing the innerseal is punched into disks, the resulting disks are glued into container caps on the caplining equipment. The cap is then applied to the container, and the capped container is then passed through an induction field to seal the innerseal to the container. In the induction heat sealing process, as the container is passed through an electromagnetic field, the field induces currents in the metallic foil, generating heat very rapidly by means of resistance heating of the foil. The heated foil causes the polymeric layer of the innerseal, which is in contact with the container mouth, to melt and form a bond upon cooling, thereby sealing the innerseal to the container mouth. In the induction field, the heated foil serves two purposes: the first is to seal the innerseal disk to the container as described above; the second is to melt the wax layer, whereby the wax is absorbed into the pulpboard layer, breaking the bond between the capliner and innerseal, thus allowing the cap to be easily removed from the container after sealing.
An innerseal that would not require induction heat sealing would be advantageous to a food packager, because it would eliminate the induction heat sealing step in the production process, thus leading to lower costs. However, because the induction sealing process also insures separation of the capliner from the innerseal, elimination of the induction sealing step would make separation of the capliner from the innerseal extremely difficult, further making it difficult to remove the cap from the container. One method of supplying a capliner and a non-induction sealable innerseal into a cap for use is by double punching, in which the capliner is first cut and glued into a cap and the innerseal is punched into the lined cap in a separate operation. This method, however, requires two steps. A material which would require neither induction heat sealing nor double punching would require the capliner and innerseal to be bonded in such a way that the capliner/innerseal composite is strong enough to be handled on conventional caplining equipment, yet will separate easily enough so that the user of the container will encounter low cap removal torques when the container is opened. Unfortunately, however, with most adhesives, the bond between the capliner and innerseal would be so strong that the cap cannot be twisted off a sealed container.